In a retrospective study the serological results from 1494 patients with community-acquired pneumonia were evaluated. An infectious etiology was found in about 40% of the cases. The majority of pneumonias was caused by Mycoplasma pneumoniae and by influenza virus type A, whereas Legionella pneumophila was the fifth most frequent pathogen. In the second part of the study, 13 hospitalized patients with community-acquired pneumonia were investigated by the whole panel of routinely used microbial methods. The etiological agent was found serologically in 3 cases and in one case by cultivation. These results suggest that the determination of serum antibodies against pathogens is frequently more useful than is generally assumed, although the yield of positive results is dependent on the epidemiological situation. The detection of elevated complement-fixing titers or specific IgM antibodies often leads to diagnosis from the first serum examined.